Google just announced some tags to help support pagination better.  They say if you have a view all option that doesn't take too long to load, searchers generally prefer that, so you can rel=canonical to that page.  However, if you don't have a view all page, then you can put these nifty rel="next" and rel="prev" tags in to let Google know your page has pagination, and where the next and previous pages are.

View all: http://googlewebmastercentral.blogspot.com/2011/09/view-all-in-search-results.html

next/prev: http://googlewebmastercentral.blogspot.com/2011/09/pagination-with-relnext-and-relprev.html

Each subdomain may have its own robots.txt file.  So for that subdomain, you can put:

User-agent: *
Disallow: /

In the robots.txt, and that should do it.

Please note that disallowing pages in robots.txt will not necessarily mean they won't appear on search result pages.... if people link to pages that are disallowed on that subdomain, they can still appear in SERPs.  I had this happen with a few pages, which leads to funny listings in the SERPs because Google has to guess what the page title and description of the page should be, since it's not allowed to read the page.  The meta noindex tag is the way to go if you want to be really sure the page doesn't appear in the SERPs.  If you use that, don't disallow the page.  Here's a recent SEOMoz post about it: http://www.seomoz.org/blog/robot-access-indexation-restriction-techniques-avoiding-conflicts

I like lowercase because when I type URLs by hand, I don't think to capitalize things.  If you capitalize things, you have to get the casing right to make the URL valid (unless you're setting up all sorts of fancy redirects), otherwise you a 404 and are left scratching your head.  Also, I agree with Dan that it looks better.

Hyphens vs. underscores is a classic question; Matt Cutts says to go with hyphens: http://www.youtube.com/watch?v=Q3SFVfDIS5k.  I like that better too.

Placing canonical tags isn't an option?  Detect that the page is being viewed through the subdomain, and if so, write the canonical tag on the page back to the root domain?

Or, just place a canonical tag on every page pointing back to the root domain (so the subdomain and root domain pages would both have them).  Apparently, it's ok to have a canonical tag on a page pointing to itself.  I haven't tried this, but if Matt Cutts says it's ok...

Add it to your site.  You want people to know the blog is part of the site, and you want people to be able to get from your site to the blog and vice versa easily.  Also, you want your site's rankings to benefit from the traffic you bring in via the blog, and vice versa.

To make it be treated as part of your site, you should set it up under a URL like mysite.com/blog, vs. blog.mysite.com.  The subdomain approach will get your blog treated like a new site.

If you think people may link to these landing pages, you'd be better off putting them on your main domain, so these pages help contribute to your main domains rank.  Generally it's better to put quality pages on your domain, instead of putting landing pages on these extra domains, and pointing them to your site.  It doesn't sound like your extra domains will have much pagerank to pass to your main domain.

Also, more importantly (for me at least), it would be strange to arrive at a landing page, and click a nav link, and arrive at a completely different domain that uses the same nav/header/footer.  I'd feel like there was some spoofing going on...

I'd just set up these extra domains to 301 redirect to my main domain, and leave it at that.

Ryan, spot on as always.

One other thing, it sounds like some of the canonicals you're placing on pages would be better suited to 301 redirects, like correcting a URL for not having a trailing slash or not.  If you can avoid using canonicals and use 301 redirects instead, that's the preferred method for resolving duplicate content issues.  Canonicals are more for when there are parameters on the URLs, and you can't get away from serving the pages with those parameters.

I don't have an answer to your question, but if you're looking for some more reading about subdomains vs. TLDs, here is a presentation given at MozCon: http://www.distilled.net/blog/seo/mozcon-international-seo/. The slideshow has some info about it, and a bunch of other good stuff.